1. Field of the Invention
The invention concerns X-ray tubes and, more particularly, a system for the suspension of a rotating anode that has only passive magnetic bearings.
2. Description of the Prior Art
X-ray tubes, for medical diagnosis for example, generally consist (FIG. 2) of a diode, namely with a cathode 21 and an anode 22 or anti-cathode, these two electrodes being enclosed in a vacuum-sealed casing 8 that enables electrical insulation between these two electrodes. The cathode 21 produces an electron beam 23 and the anode 22 receives the electrons on a small area forming a focal spot from which the X-rays are emitted.
When the high supply voltage is applied by a generator 24 to the terminals of the cathode 21 and the anode 22 so that the cathode is at the negative potential (-HT), a so-called anode current is set up in the circuit through the generator 24 producing the high supply voltage. The anode current flows through the space between the cathode and the anode in the form of the beam of electrons 23 which bombard the focal spot.
A small proportion of the energy used to produce the beam of electrons 23 is converted into X-rays while the rest of this energy is converted into heat. Thus, given also the high instantaneous power values (of the order of 100 KW) and the small dimensions of the focal spot (of the order of one millimeter), manufacturers have long been making X-ray tubes with rotating anodes where the anode is made to rotate to distribute the thermal flux on a crown or ring called a focal ring with a far greater area than the focal spot, the interest being all the greater as the rotation speed is high (generally between 3,000 and 12,000 rpm).
The standard type of rotating anode has the general shape of a disk with an axis of symmetry 4 around which it is made to rotate by means of an electrical motor 1. The electrical motor has a stator 2 located outside the casing 8 and a rotor 3 mounted in the casing of the X-ray tube and placed along the axis of symmetry, the rotor being mechanically and solidly joined to the anode by means of a supporting shaft 25.
It is becoming increasingly frequent to use permanently working X-ray tubes. This leads to ever greater levels of dissipated energy. To cool the anode more efficiently, it is made to rotate at ever higher rotation speeds of the order of 10,000 rpm in mechanical type tubes, the rotor of which is mounted on ball bearings. These ball bearings have to work under very difficult conditions, for the temperature is high and the high vacuum of the tube in which they are placed prevents lubrication by a liquid.
Furthermore, a mechanical system of this type generates noise and vibrations that are detrimental to the comfort of the practioner and the patient as well as to the quality of the image obtained.
To overcome the drawbacks of mechanical bearing systems, it has been proposed to use magnetic devices to suspend the rotor/anode set, thus preventing any mechanical friction.
It is recalled that, to maintain a body 30, with a cylindrical shape generated by revolution and an axis O.sub.1 Z (FIG. 1), it is necessary to control over 5 degrees of freedom along the axis O.sub.1 X.sub.1, O.sub.1 Y.sub.1, O.sub.2 X.sub.2, O.sub.2 Y.sub.2 and O.sub.1 Z, with the body retaining the capacity to rotate around the axis O.sub.1 Z.
According to Ernshaw's Theorem, it is impossible to obtain complete magnetic levitation of a body, namely without any physical contact, and in a purely passive manner that is, by using only permanent magnets. This is why all hitherto proposed devices with magnetic bearings for the rotation of a rotor/anode set of an X-ray tube, have at least one active channel made with an electromagnet associated with an electronic servo-control circuit positiond along the axis corresponding to this active channel.
This, therefore, leads to the making of a fairly complicated active system to maintain, for example, the position of the rotor along the axis O.sub.1 Z. To this end, it is necessary to have position detectors. Information on positional deviation, with respect to these detectors, controls the value of the current in the electromagnet so as to reduce this deviation.
An aim of the present invention is to make a system for the suspension of an X-ray tube rotating anode of the type having passive magnetic bearings, wherein the maintaining of the longitudinal position of the rotor along the axis does not require the use of a complicated servo-control system.